The invention relates to a system for positioning an output of a video display.
Continual advances in computer technology are making possible cost-effective, yet high performance computers capable of displaying high resolution images. A variety of display devices, including cathode ray tube (CRT) displays or thin-film-transistor (TFT) flat panel displays, may be used. These displays are driven by graphics peripherals such as video cards, which in turn are controlled by processors inside the computers.
Traditionally, due to their high cost, flat panel displays have been used only in notebook computers where size and low power requirements are important. As a notebook computer can drive its built-in flat panel display as well as an external CRT display, the video circuit of the notebook computer automatically handles differences between the CRT display and the flat panel display. In the controlled environment of the notebook computer, the maximum resolution of a flat panel display controller may be greater than or equal to that of the notebook computer's built-in flat panel display. However, in a desktop computer where a particular display attached to the desktop computer may be changed by a user, potential incompatibilities exist when a high resolution flat panel display is used with a desktop computer which is capable of driving only a low resolution flat panel display. For example, certain flat panel capable desktop computers are currently equipped with 65 megahertz (MHZ) video outputs and are limited to a resolution of 1024.times.768 pixels. When higher resolution flat panel displays become available, these flat panel displays may be incompatible with the original display circuits.
Computer systems capable of supporting both CRT displays as well as flat panel displays need to handle differences between the two types of display. For example, in the CRT display, an electron beam is swept horizontally across a line of the screen and, at the end of the line, the electron beam is moved vertically down to the next line before the horizontal sweep motion is repeated. Upon reaching the end of the screen, the electron beam is moved back to the origin of the screen and the process is repeated. These timing requirements are referred to as horizontal and vertical retrace timing requirements. The flat panel displays do not require as much time for horizontal and vertical blanking since they are digital devices which are addressed via internal counters and latches instead of an electron beam sweeping motion.
Also, each CRT display inherently has a variable resolution and accepts multiple input resolutions. "Multi-sync" circuitry is used to respond to video signals to control the CRT raster scan frequency. The CRT display's control signals include vertical sync (VSYNC), horizontal sync (HSYNC), and RED, GREEN, BLUE (RGB) signals. The HSYNC and VSYNC are signals defining horizontal and vertical raster frequency which are synchronized with the CRT display's logic. RED, GREEN and BLUE are analog signals which contain color data for each pixel. In contrast, the flat panel display operates at a fixed resolution and is controlled by video signals HSYNC, VSYNC, PIXCLOCK, RGBPIXDATA, and DATA_ENABLE. HSYNC and VSYNC are digital signals which provide similar function as the same named signals on a CRT interface. RGBPIXDATA is the digital RGB data and is typically 18-24 bits for each pixel. DATA_ENABLE identifies valid pixel data, which are latched with a pixel clock signal, PIXCLOCK.
Also, a flat panel display controller typically has a maximum resolution limited by a maximum clock frequency supported from its video output circuitry. The controller supports resolutions below the fixed resolution of its flat panel display via circuitry in the flat panel display controller. This circuitry provides at a minimum the ability to center a low resolution display on the panel. Other flat panel display controllers provide circuitry for upscaling the low resolution to the high native resolution of the panel using either pixel replication, or line replication, or interpolation with filtering at various quality levels. However, incompatibilities may exist between the display controller in the computer and the controller in the flat panel display and which may affect the display quality.